This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Nanocrystalline (NC) materials, with average and range of grain sizes typically smaller than 100 nm, have attracted more and more attention from the materials community for decades. Contrary to conventional coarse-grained counterparts, NC materials exhibit peculiar and interesting mechanical, physical and chemical properties such as, but not limited to, increased mechanical strength, enhanced diffusivity and higher specific heat. Due to these peculiar and interesting properties, NC materials are experiencing a rapid development in recent years for their existing and/or potential applications in a wide variety of technological areas such as electronics, catalysis, batteries, magnetic data storage, structural components and so on.
Conventional coarse-grained metal powders and flakes have been widely used in the surface coating technology and polymer composites. It has been proved that metal powders and flakes could improve the wear resistance, corrosion resistance, and scratch resistance of the coatings. In addition, metal flakes could be utilized as conductive filler to produce polymer composites used as shields against electromagnetic interference and electrically conducting thermoplastic composites. Due to the significant increase in hardness, strength and electrical conductivity of NC metals and alloys, metal powders and flakes with nanocrystalline structure hold promise for engineering applications, especially in fields such as surface coatings, polymer composites, etc.
Methods for generating NC metals and alloys generally include severe plastic deformation (SPD), mechanical alloying, electrode position, and sputtering. As one of the SPD methods, ultrasonic shot peening (USP), i.e., shot peening driven using high intensity ultrasonic vibration has the advantage of high efficiency and has been successfully used in forming nanostructures at the surface of a metallic workpiece, subjected to USP. As one of the SPD methods, ultrasonic shot peening (USP), or shot peening driven using high intensity ultrasonic vibration, has the advantage of high efficiency and has been successfully used in forming nanostructures at the surface of a metallic workpiece subjected to USP. Several published papers indicate that NC materials could be successfully generated via USP in pure iron, copper and other metals and alloys. Previously, a layer consisting of NC materials at the surface of an ultrasonic shot peened sample has been successfully generated. The research results indicated that nanograins in the size of 100 nm and nanocrystalline surface layer with the thickness of 1 μm were fabricated after USP treatment of 20 minutes. By increasing the USP treatment duration, nanograins in the size of 20 nm and nanocrystalline surface layer with the thickness of 10-20 μm were successfully produced. It should be recognized that such nanostructures were on the surface of the bulk material and were not separable from the workpiece during the manufacturing process. For the purposes of this disclosure nanograins are to be understood to be grains whose size is typically leas than about 500 nm. However, it should be understood this is not a rigid limit.
Methods capable of producing flakes or powders consisting of polycrystalline nanostructures include ball milling and rapid solidification of small liquid droplets (metallic glass) followed by annealing/heat treatment (crystallization). Large metallic powders are ball milled for many hours or even days to create nanostructures in the powders. This method, however, suffers from contamination from the interactions between the powders and the balls or the internal walls of the container. Rapid solidification methods can also lead to surface contamination during quenching of droplets.
Thus, there is unmet need to produce nanocrystalline metal powders and flakes from polycrystalline aggregates without the disadvantages of long time, high energy consumption, and contamination issues.